syllabus

INCREASE IN SCALABILITY OF DWDM TRANSMISSION USING

FORWARD ERROR CORRECTION

By

S.IMMANUEL

(Reg.No:1026005)

A PROJECT REPORT

Submitted to the

FACULTY OF INFORMATION AND COMMUNICATION ENGINEERING

In partial fulfillment of the requirements

For the award of the degree

of

MASTER OF ENGINEERING

IN

OPTICAL COMMUNICATION

A.C. COLLEGE OF ENGINEERING AND TECHNOLOGY,

KARAIKUDI-4.

ANNA UNIVERSITY

CHENNAI 600 025

MAY- 2012

ii

DECLARATION

I hereby declare that the work entitled “ INCREASE IN SCALABILITY

OF DWDM TRANSMISSION USING FORWARD ERROR CORRECTION” is

submitted in partial fulfillment of the requirement for the award of the degree in M.E.,

Anna University, Chennai- 600 025, is a record of the my own work carried out by me

during the academic year 2011 – 2012 under the supervision and guidance of

Dr.A.Sivanantha Raja, Associate Professor, Department of Electronics and

Communication Engineering, Alagappa Chettiar College of Engineering and Technology,

Karaikudi. The extent and source of information are derived from the existing literature

and have been indicated through the dissertation at the appropriate places. The matter

embodied in this work is original and has not been submitted for the award of any other

degree or diploma, either in this or any other University.

S.Immanuel

Reg. No.1026005

I certify that the declaration made above by the candidate is true

Dr.A.Sivanantha Raja,

Associate Professor,

Department of ECE

iii

BONAFIDE CERTIFICATE

Certified that this project report titled “INCREASE IN SCALABILITY OF

DWDM TRANSMISSION USING FORWARD ERROR CORRECTION” is the bonafide

work of Mr. S.IMMANUEL, Reg.No.1026005 who carried out the research under my

supervision. Certified further, that to the best of my knowledge the work reported herein

does not form part of any other project report or dissertation on the basis of which a degree

or award was conferred on an earlier occasion on this or any other candidate.

SIGNATURE

Dr. A.Sivanantha Raja

SUPERVISOR

Forwarded by

SIGNATURE

Prof.I.Muthumani M.E.,

HEAD OF THE DEPARTMENT

Examined on:

Internal Examiner External Examiner

iv

ABSTRACT

The scalability of spectrum sliced dense wavelength-division-multiplexing

(DWDM) transmission systems primarily intended for metro access applications. A

theoretical analysis elucidates the tradeoff between the loss budget and the sliced

bandwidth (i.e., the number of channels with assuming light sources with a fixed

bandwidth). Moreover, the use of forward error correction (FEC) to expand scalability is

studied. Based on the analysis, two spectrum-sliced DWDM transmission schemes are

introduced. One demonstrates 10-Gb/s, eight channel spectrum-sliced DWDM

transmission with the channel spacing of 200 GHz without FEC, and the other confirms

10-Gb/s, eight channel spectrum-sliced DWDM transmission with the channel spacing of

200 GHz with FEC. We have also conducted 100 GHZ Grid 40 Channel, 50 GHZ Grid 80

Channel, 25 GHZ Grid 160 Channel DWDM Transmission without FEC. We have also

conducted 100 GHZ Grid 40 Channel, 50 GHZ Grid 80 Channel, 25 GHZ Grid 160

Channel DWDM Transmission with FEC.

v

ACKNOWLEDGEMENT

I thank god for having gracefully blessed us to come up till now and thereby giving

strength and courage to complete the project successfully. I sincerely submit this project to

the almighty lotus feet.

With profound gratitude, respect and pride I express my sincere thanks to our

principal Dr.P.N.NEELAKANTAN, for his encouragement and keen interest shown in

my project.

It gives me great pleasure to express my sincere gratitude to Head of Department

Prof.I.MUTHUMANI, for her constant support and encouragement to me for completing

this project work.

I owe deep depth of gratitude to my beloved guide Dr.A.SIVANANTHA RAJA,

for his inspiration and guidance and lending all assistants and support at each and every

stage, which made me to complete this project in an efficient and successful manner.

I wish to thank Prof.K.KALAISELVI.,M.E., and Prof.R.SAROJINI.,M.E.,

Department of ECE and Mr.A.Vetrivel SDE(BSNL) for their suggestions and help during

this project period.

I happily acknowledge my family members and friends for their support lend to me

with which, I have completed this endeavor.

vi

CONTENTS

CHAPTER

NO.

TITLE PAGE

NO

.

1 INTRODUCTION 1

1.1 MOTIVATION FOR THE PROJECT 1

1.2 LITERATURE REVIEW

1

1.3 OPTICAL FIBERS

3

1.4 OPTICAL FIBER COMMUNICATION

4

1.5 PRINCIPLE OF OPERATION

5

1.6 INDEX OF REFRACTION

5

1.7 TYPES OF OPTICAL FIBERS

5

1.7.1 MULTI MODE FIBER

5

1.7.2 SINGLE MODE FIBER

7

1.8 MECHANISM OF ATTENUATION

8

1.9 TOTAL INTERNAL REFRACTION

9

1.10 TERMINATION AND SPLICING

9

2 WAVELENGTH DIVISION MULTIPLEXING 12

2.1 WDM SYSTEMS 12

2.2 COARSE WAVELENGTH DIVISION MULTIPLEXING

14

2.3 DENSE WAVELENGTH DIVISION MULTIPLEXING

15

3 FIBER USED IN DWDM 17

3.1 NONZERO DISPERSION SHIFTED FIBER (ITU-T G.655) 17

3.2 RECOMMENDATION G.655

17

4 DWDM SYSTEMS 19

vii

4.1 MANAGING THE DWDM NETWORK 21

4.1.1 WAVELENGTH CONVERTING TRANSPONDERS 21

4.1.2 MUXPONDER 23

4.1.3 RECONFIGURABLE ADD DROP MULTIPLEXER

(ROADM)

23

4.1.4 OPTICAL CROSS CONNECTS (OXCS) 23

5 PRACTICAL DWDM SYSTEM 24

5.1 DWDM SETUP 24

5.1.1 COMPONENTS IN DWDM

24

5.1.2 PROCEDURE FOR SETUP 25

6 FORWARD ERROR CORRECTION 26

6.1 TYPES OF FEC 26

6.1.1 LIST OF FEW ERROR CORRECTING CODES 27

6.2 FEC IN OPTICAL NETWORK 27

7 SPECTRUM SCLICED DWDM TRANSMISSION 29

7.1 CONFIGURATION OF TARGET SYSTEMS 29

7.2 THEORETICAL ANALYSIS 30

7.2.1 SCALABILITY: TRADEOFF BETWEEN LOSS BUDGET

AND CHANNEL SPACING

30

7.2.2 USE OF FEC FOR EXPANDING THE SCALABILITY 32

8 DWDM EXPERIMENTS 38

8.1 10-G SPECTRUM SCLICED DWDM TRANSMISSION USING

FEC

38

9 RESULTS, ANALYSIS AND DISCUSSIONS 40

10 CONCLUSION AND FUTURE SCOPE 47

viii

10.1 CONCLUSION 47

10.2 FUTURE SCOPE 47

REFERENCES 48

PUBLICATIONS 50

ix

LIST OF FIGURES

FIGURE

NO.

TITLE PAGE

NO.

1.1 PROPAGATION OF LIGHT THROUGH A MULTI-MODE OPTICAL

FIBER

5

1.2 STRUCTURE OF TYPICAL SINGLE MODE FIBER 7

1.3 LIGHT ATTENUATION BY ZBLAN AND SILICA FIBER 8

1.4 ST CONNECTORS ON MULTI MODE FIBER 10

2.1 CWDM BANDWIDTH 14

2.2 DWDM BANDWIDTH 16

3.1 1550 NM BAND PERFORMANCE 18

4.1 DWDM TRANSMISSION 20

5.1 DWDM SETUP 24

7.1 MODEL FOR THEORETICAL ANALYSIS 29

7.2 CALCULATED LOSS BUDGET TO ACHIEVE THE BER OF 1E-12

WITHOUT FEC (1.25 GB/S PER CHANNEL).

31

7.3 CALCULATED LOSS BUDGET TO ACHIEVE THE BER OF 1 ×

10−12 WITHOUT FEC (10 GB/S PER CHANNEL).

32

7.4 CALCULATED LOSS BUDGET TO ACHIEVE THE BER OF 1E−12

WITH FEC (10 GB/S PER CHANNEL).

33

7.5 EXPANSION OF SCALABILITY PERMITTED BY USING FEC. 34

7.6 EXPERIMENTAL SETUP FOR CONFIRMING THE RESULTS OF

THEORETICAL ANALYSIS.

35

7.7 BER CHARACTERISTICS WHEN SLICED BANDWIDTH IS 130/200

GHZ.

36

8.1(1) EXPERIMENTAL SETUP FOR 10 G × EIGHT CH DWDM

TRANSMISSION;

38

8.1(2) MEASURED OPTICAL SPECTRUM AFTER MULTIPLEXING. 38

9.1 BER CHARACTERISTICS OF 10 GB/S EIGHT CHANNEL DWDM 40

x

TRANSMISSION WITHOUT FEC.

9.2 BER CHARACTERISTICS OF 10 GB/S EIGHT CHANNEL DWDM

TRANSMISSION WITH FEC.

41

9.3 BER CHARACTERISTICS OF 40 CHANNEL 100GHZ SPACING

DWDM TRANSMISSION WITHOUT FEC.

42

9.4 BER CHARACTERISTICS OF 40 CHANNEL 100GHZ SPACING

DWDM TRANSMISSION WITH FEC

42

9.5 BER CHARACTERISTICS OF 80 CHANNEL 50GHZ SPACING

DWDM TRANSMISSION WITHOUT FEC

43

9.6 BER CHARACTERISTICS OF 80 CHANNEL 50GHZ SPACING

DWDM TRANSMISSION WITH FEC

44

9.7 BER CHARACTERISTICS OF 160 CHANNEL 25GHZ SPACING

DWDM TRANSMISSION WITHOUT FEC

44

9.8 BER CHARACTERISTICS OF 160 CHANNEL 25GHZ SPACING

DWDM TRANSMISSION WITH FEC

45

xi

LIST OF TABLES

TABLE

NO.

TITLE PAGE

NO.

1.1 SIZE OF DIFFERENT MEDIUM 8

9.1 BER VALUES OF VARIOUS DWDM TRANSMISSION SYSTEM 46

CHAPTER 1

INTRODUCTION

1.1 MOTIVATION FOR THE PROJECT

Network operators project a long term trend of traffic growth at a rate of over 75％

per year, which in turn requires the capacity to be doubled approximately every 12-18

months. Given the current network traffic load and growth rate of the operators, there

exists an urgent need to increase the network capacity through improvements in spectral

efficiency. This can be accomplished by launching 100Gb/s per channel instead of 10Gb/s

over 50GHz spaced channels.

One of the most cost effective architectures is to deploy 100Gb/s systems utilizing

existing 10Gb/s infrastructure. Long distance DWDM communication systems are

typically limited by optical signal-to-noise ratio (OSNR). Unfortunately, a straight forward

10x increase in the data rate over an existing channel results in a 10x reduction in OSNR.

Closing this significant performance gap between the two systems requires a 10x

improvement in OSNR for 100Gb/s implementations. There are several techniques that can

be used to reduce the OSNR deficit such as through DP-QPSK modulation in conjunction

with a coherent receiver. However, even after taking advantage of all these techniques, a

significant OSNR deficit remains. The OSNR gap must be closed in order to achieve the

objective of transmitting 100Gb/s over existing 10Gb/s infrastructure. Among the available

technologies to further improve the OSNR deficit, Forward Error Correction (FEC) is

commonly considered as an attractive cost-effective candidate to recover the lost

sensitivity due to the transition to higher data rates.

1.2 LITERATURE REVIEW

Development of “ INCREASE IN SCALABILITY OF DWDM

TRANSMISSION USING FORWARD ERROR CORRECTION” was carried out

according to the Literature Survey Performed as given below

Takashi Mitsui, Kazutaka Hara, Masamichi Fujiwara, Jun-ichi Kani Masashi

Tadokoro,Naoto Yoshimoto, and Hisaya Hadama have showed that the sensitivity

was improved by -31.5 dB m and the loss budget was obtained about 20 dB at a sliced

1

bandwidth of 400 GHz by using FEC even if assuming a light source of 0 dB m output

power[1].

S. Kaneko et al have showed that FEC increases not only the loss budget to

match those of normal WDM systems, but also the number of available channels, which is

a feature not possible in normal WDM systems[2].

Spectral grids for WDM applications: DWDM frequency grid,‖ ITU-T

Recommendation G.694.1, K. Fukuchi, T. Kasamatsu, M. Morie, R. Ohhira, T. Ito,

K. Sekiya, D. Ogasahara, and T. Ono, R. D. Feldman, E. E. Harstead, S. Jiang, T. H.

Wood, and M. Zirngibl, K. Iwatsuki, J. Kani, H. Suzuki, and M. Fujiwara, have

showed that Dense wavelength-division-multiplexing (DWDM) [3] is the key technology

to increase the capacity of optical fiber transmission, and so has been widely deployed to

core networks to cope with the increasing demand to transfer huge loads such as Internet

traffic. As the next step, while further increases in capacity (e.g., over 10 Tb/s) have been

pursued [4],several attempts have been made to apply DWDM to access networks [5], [6].

J. S. Lee, Y. C. Chung, and D. J. DiGiovanni, have proposed that The most

important issue in DWDM access networks is to decrease the burden of

operating/administrating ―wavelengths,‖ which may be assigned differently to each user

or each user group in the access networks [7].

J. S. Lee, Y. C. Chung, and D. J. DiGiovanni, K. Akimoto, J. Kani, M.

Teshima, and K. Iwatsuki, J. H. Han, S. J. Kim, and J. S. Lee, S. J. Kim, J. H. Han, J.

S. Lee, and C. S. Park, K. H. Han, E. S. Son, H. Y. Choi, K. W. Lim, and Y. C.

Chung, have showed that The spectrum-sliced wavelength division multiplexing (WDM) scheme has

been proposed as one of the approaches to address this issue [7]–[11].

J.S. Lee, Y. C. Chung, and D. J. DiGiovanni, have showed that a particular

noise factor in the spectrum-sliced scheme is the signal-signal beat noise; the signal-to-

noise ratio (SNR) can be expressed as being proportional to the ratio of data rate to sliced

bandwidth [7].

J. H. Han, S. J. Kim, and J. S. Lee, S. J. Kim, J. H. Han, J. S. Lee, and C. S.

Park, have showed that 2.5-Gb/s spectrum-sliced transmission in which the bandwidth

2

was broadened through intra channel four-wave mixing in a nonlinear fiber at the receiver

[9] or a gain saturated semiconductor optical amplifier (SOA) at the transmitter [10].

K. H. Han, E. S. Son, H. Y. Choi, K. W. Lim, and Y. C. Chung, have showed

that the increase in scalability offered by forward error correction (FEC) is detailed. An

attempt to increase the loss budget by implementing FEC to spectrum-sliced WDM

systems was reported in [11].

1.3 OPTICAL FIBERS

An optical fiber is a thin, flexible, transparent fiber that acts as a waveguide, or

"light pipe", to transmit light between the two ends of the fiber. The field of applied

science and engineering concerned with the design and application of optical fibers is

known as fiber optics. Optical fibers are widely used in Fiber Optic Communications,

which permits transmission over longer distances and at higher bnadwidths (data rates)

than other forms of communication. Fibers are used instead of metal wires because signals

travel along them with less loss and are also immune to electromagnetic interference.

Fibers are also used for illumination, and are wrapped in bundles so they can be used to

carry images, thus allowing viewing in tight spaces. Specially designed fibers are used for

a variety of other applications, including sensors and fiber lasers.

Optical fiber typically consists of a transparent core surrounded by a transparent

cladding material with a lower index of refraction. Light is kept in the core by total internal

refraction. This causes the fiber to act as a waveguide. Fibers which support many

propagation paths or transverse modes are called multi mode fibers ,while those which can

only support a single mode are called single mode fibers. Multi-mode fibers generally have

a larger core diameter, and are used for short-distance communication links and for

applications where high power must be transmitted. Single-mode fibers are used for most

communication links longer than 1,050 meters. Joining lengths of optical fiber is more

complex than joining electrical wire or cable. The ends of the fibers must be carefully

cleaved, and then spliced together either mechanically or by fusing them together with

heat. Special optical fiber connectors are used to make removable connections.

3

1.4 OPTICAL FIBER COMMUNICATION

Optical fiber can be used as a medium for telecommunication and networking

because it is flexible and can be bundled as cables. It is especially advantageous for long-

distance communications, because light propagates through the fiber with little attenuation

compared to electrical cables. This allows long distances to be spanned with few repeaters.

Additionally, the per-channel light signals propagating in the fiber have been modulated at

rates as high as 111 gigabits per second by NTT, although 10 or 40 Gbit/s is typical in

deployed systems. Each fiber can carry many independent channels, each using a different

wavelength of light (Wavelength Division Multiplexing (WDM)). The net data rate (data

rate without overhead bytes) per fiber is the per-channel data rate reduced by the FEC

overhead, multiplied by the number of channels (usually up to eighty in commercial dense

WDM systems as of 2008). The current laboratory fiber optic data rate record, held by Bell

Labs in Villarceaux, France, is multiplexing 155 channels, each carrying 100 Gbit/s over a

7000 km fiber. Nippon Telegraph and Telephone Corporation have also managed 69.1

Tbit/s over a single 240 km fiber (multiplexing 432 channels, equating to 171 Gbit/s per

channel). Bell Labs also broke a 100 Petabit per second kilometer barrier (15.5 Tbit/s over

a single 7000 km fiber)

For short distance applications, such as creating a network within an office

building, fiber-optic cabling can be used to save space in cable ducts. This is because a

single fiber can often carry much more data than many electrical cables, such as 4 pair cat-

5 Ethernet cabling. Fiber is also immune to electrical interference; there is no cross-talk

between signals in different cables and no pickup of environmental noise. Non-armored

fiber cables do not conduct electricity, which makes fiber a good solution for protecting

communications equipment located in high voltage environments such as power generation

facilities, or metal communication structures prone to lightning strikes. They can also be

used in environments where explosive fumes are present, without danger of ignition.

Wiretapping is more difficult compared to electrical connections, and there are concentric

dual core fibers that are said to be tap-proof.

4

1.5 PRINCIPLE OF OPERATION

An optical fiber is a cylindrical dielectric waveguide (non-conducting waveguide)

that transmits light along its axis, by the process of total internal reflection. The fiber

consists of a core surrounded by a cladding layer, both of which are made of dielectric

materials. To confine the optical signal in the core, the refractive index of the core must be

greater than that of the cladding. The boundary between the core and cladding may either

be abrupt, in step-index fiber, or gradual, in graded-index fiber.

1.6 INDEX OF REFRACTION

The index of refraction is a way of measuring the speed of light in a material. Light

travels fastest in a vacuum, such as outer space. The speed of light in a vacuum is about

300,000 kilometers (186 thousand miles) per second. Index of refraction is calculated by

dividing the speed of light in a vacuum by the speed of light in some other medium. The

index of refraction of a vacuum is therefore 1, by definition. The typical value for the

cladding of an optical fiber is 1.46. The core value is typically 1.48. The larger the index of

refraction, the slower light travels in that medium. From this information, a good rule of

thumb is that signal using optical fiber for communication will travel at around 200 million

meters per second. Or to put it another way, to travel 1000 kilometers in fiber, the signal

will take 5 milliseconds to propagate. Thus a phone call carried by fiber between Sydney

and New York, a 12000 kilometer distance, means that there is an absolute minimum delay

of 60 milliseconds (or around 1/16 of a second) between when one caller speaks to when

the other hears.

1.7 TYPES OF OPTICAL FIBERS

1.7.1 Multimode Fiber

Fig 1.1 Propagation of light through a multi-mode optical fiber

5

Fiber with large core diameter (greater than 10 micrometers) may be analyzed by

geometrical optics. Such fiber is called multi-mode fiber, from the electromagnetic analysis

(see below). In a step-index multi-mode fiber, rays of light are guided along the fiber core

by total internal reflection. Rays that meet the core-cladding boundary at a high angle

(measured relative to a line normal to the boundary), greater than the critical angle for this

boundary, are completely reflected. The critical angle (minimum angle for total internal

reflection) is determined by the difference in index of refraction between the core and

cladding materials. Rays that meet the boundary at a low angle are refracted from the core

into the cladding, and do not convey light and hence information along the fiber. The

critical angle determines the acceptance angle of the fiber, often reported as a numerical

aperture. A high numerical aperture allows light to propagate down the fiber in rays both

close to the axis and at various angles, allowing efficient coupling of light into the fiber.

Rays that meet the boundary at a low angle are refracted from the core into the cladding,

and do not convey light and hence information along the fiber. However, this high

numerical aperture increases the amount of dispersion as rays at different angles have

different path lengths and therefore take different times to traverse the fiber.

In graded-index fiber, the index of refraction in the core decreases continuously

between the axis and the cladding. This causes light rays to bend smoothly as they

approach the cladding, rather than reflecting abruptly from the core-cladding boundary.

The resulting curved paths reduce multi-path dispersion because high angle rays pass more

through the lower-index periphery of the core, rather than the high-index center. This

causes light rays to bend smoothly as they approach the cladding, rather than reflecting

abruptly from the core-cladding boundary. The index profile is chosen to minimize the

difference in axial propagation speeds of the various rays in the fiber. This ideal index

profile is very close to a parabolic relationship between the index and the distance from the

axis.

6

1.7.2 Single Mode Fiber

Fig 1.2 Structure of Typical Single Mode Fiber

Fiber with a core diameter less than about ten times the wavelength of the

propagating light cannot be modeled using geometric optics. Instead, it must be analyzed

as an electromagnetic structure, by solution of Maxwell's equations as reduced to the

electromagnetic wave equation. The electromagnetic analysis may also be required to

understand behaviors such as speckle that occur when coherent light propagates in multi-

mode fiber. As an optical waveguide, the fiber supports one or more confined transverse

modes by which light can propagate along the fiber. Fiber supporting only one mode is

called single-mode or mono-mode fiber. The behavior of larger-core multi-mode fiber can

also be modeled using the wave equation, which shows that such fiber supports more than

one mode of propagation (hence the name). The results of such modeling of multi-mode

fiber approximately agree with the predictions of geometric optics, if the fiber core is large

enough to support more than a few modes.

The waveguide analysis shows that the light energy in the fiber is not completely

confined in the core. Instead, especially in single-mode fibers, a significant fraction of the

energy in the bound mode travels in the cladding as an evanescent wave.

The most common type of single-mode fiber has a core diameter of 8–10

micrometers and is designed for use in the near infrared. The mode structure depends on

the wavelength of the light used, so that this fiber actually supports a small number of

additional modes at visible wavelengths. Multi-mode fiber, by comparison, is

7

manufactured with core diameters as small as 50 micrometers and as large as hundreds of

micrometers. The normalized frequency V for this fiber should be less than the first zero of

the Bessel functionJ0 (approximately 2.405).

TABLE 1.1

Size of Different Medium

S.No. Medium Diameter

1 Core 8 µm

2 Cladding 125 µm

3 Buffer 250 µm

4 Jacket 400 µm

1.8 MECHANISM OF ATTENUATION

Fig 1.3 Light attenuation by ZBLAN and silica fibers

Attenuation in fiber optics, also known as transmission loss, is the reduction in

intensity of the light beam (or signal) with respect to distance traveled through a

transmission medium. Attenuation coefficients in fiber optics usually use units of dB/km

8

through the medium due to the relatively high quality of transparency of modern optical

transmission media. The medium is usually a fiber of silica glass that confines the incident

light beam to the inside. Attenuation is an important factor limiting the transmission of a

digital signal across large distances. Thus, much research has gone into both limiting the

attenuation and maximizing the amplification of the optical signal. Empirical research has

shown that attenuation in optical fiber is caused primarily by both scattering and

absorption.

1.9 TOTAL INTERNAL REFRACTION

When light traveling in a dense medium hits a boundary at a steep angle (larger

than the "critical angle" for the boundary), the light will be completely reflected. This

effect is used in optical fibers to confine light in the core. Light travels along the fiber

bouncing back and forth off of the boundary. Because the light must strike the boundary

with an angle greater than the critical angle, only light that enters the fiber within a certain

range of angles can travel down the fiber without leaking out. This range of angles is called

the acceptance cone of the fiber. The size of this acceptance cone is a function of the

refractive index difference between the fiber's core and cladding.

In simpler terms, there is a maximum angle from the fiber axis at which light may

enter the fiber so that it will propagate, or travel, in the core of the fiber. The sine of this

maximum angle is the numerical aperture (NA) of the fiber. Fiber with a larger NA

requires less precision to splice and work with than fiber with a smaller NA. Single-mode

fiber has a small NA.

1.10 TERMINATION AND SPLICING

Optical fibers are connected to terminal equipment by optical fiber connectors.

These connectors are usually of a standard type such as FC, SC, ST, LC, or MTRJ.

9

Fig 1.4 ST Connectors on Multi Mode Fiber

Optical fibers may be connected to each other by connectors or by splicing, that is,

joining two fibers together to form a continuous optical waveguide. The generally accepted

splicing method is arc fusion splicing, which melts the fiber ends together with an electric

arc. For quicker fastening jobs, a "mechanical splice" is used.

Fibers are terminated in connectors so that the fiber end is held at the end face

precisely and securely. A fiber-optic connector is basically a rigid cylindrical barrel

surrounded by a sleeve that holds the barrel in its mating socket. The mating mechanism

can be "push and click", "turn and latch" ("bayonet"), or screw-in (threaded). A typical

connector is installed by preparing the fiber end and inserting it into the rear of the

connector body. Quick-set adhesive is usually used so the fiber is held securely, and a

strain relief is secured to the rear. Once the adhesive has set, the fiber's end is polished to a

mirror finish. Various polish profiles are used, depending on the type of fiber and the

application. For single-mode fiber, the fiber ends are typically polished with a slight

curvature, such that when the connectors are mated the fibers touch only at their cores.

This is known as a "physical contact" (PC) polish. The curved surface may be polished at

an angle, to make an "angled physical contact" (APC) connection. Such connections have

higher loss than PC connections, but greatly reduced back reflection, because light that

10

reflects from the angled surface leaks out of the fiber core; the resulting loss in signal

strength is known as gap loss. APC fiber ends have low back reflection even when

disconnected.

11

CHAPTER 2

WAVELENGTH DIVISION MULTIPLEXING

In fiber-optic communications, wavelength-division multiplexing(WDM) is a

technology which multiplexes a number of optical carrier signals onto a single optical fiber

by using different wavelengths (colours) of laser light. This technique enables bidirectional

communications over one strand of fiber, as well as multiplication of capacity.

The term wavelength-division multiplexing is commonly applied to an optical

carrier (which is typically described by its wavelength), whereas frequency-division

multiplexing typically applies to a radio carrier (which is more often described by

frequency). Since wavelength and frequency are tied together through a simple directly

inverse relationship, the two terms actually describe the same concept.

2.1 WDM SYSTEMS

A WDM system uses a multiplexer at the transmitter to join the signals together,

and a de-multiplexer at the receiver to split them apart. With the right type of fiber it is

possible to have a device that does both simultaneously, and can function as an optical

add-drop multiplexer. The optical filtering devices used have traditionally been etalons,

stable solid-state single-frequency Fabry–Pérot interferometers in the form of thin-film-

coated optical glass.

The concept was first published in 1970, and by 1978 WDM systems were being

realized in the laboratory. The first WDM systems only combined two signals. Modern

systems can handle up to 160 signals and can thus expand a basic 10 Gbit/s system over a

single fiber pair to over 1.6 Tbit/s.

WDM systems are popular with telecommunications companies because they allow

them to expand the capacity of the network without laying more fiber. By using WDM and

optical amplifiers, they can accommodate several generations of technology development

in their optical infrastructure without having to overhaul the backbone network. Capacity

of a given link can be expanded by simply upgrading the multiplexers and demultiplexers

at each end.

12

This is often done by using optical-to-electrical-to-optical (O/E/O) translation at the

very edge of the transport network, thus permitting interoperation with existing equipment

with optical interfaces.

Most WDM systems operate on single-mode fiber optical cables, which have a core

diameter of 9 µm. Certain forms of WDM can also be used in multi-mode fiber cables

(also known as premises cables) which have core diameters of 50 or 62.5 µm.

Early WDM systems were expensive and complicated to run. However, recent

standardization and better understanding of the dynamics of WDM systems have made

WDM less expensive to deploy.

Optical receivers, in contrast to laser sources, tend to be wideband devices.

Therefore the de-multiplexer must provide the wavelength selectivity of the receiver in the

WDM system.

WDM systems are divided in different wavelength patterns, conventional or coarse

and dense WDM. Conventional WDM systems provide up to 8 channels in the 3rd

transmission window (C-Band) of silica fibers around 1550 nm. Dense wavelength

division multiplexing (DWDM) uses the same transmission window but with denser

channel spacing. Channel plans vary, but a typical system would use 40 channels at

100 GHz spacing or 80 channels with 50 GHz spacing. Some technologies are capable of

25 GHz spacing (sometimes called ultra-dense WDM). New amplification options (Raman

amplification) enable the extension of the usable wavelengths to the L-band, more or less

doubling these numbers.

Coarse wavelength division multiplexing (CWDM) in contrast to conventional

WDM and DWDM uses increased channel spacing to allow less sophisticated and thus

cheaper transceiver designs. To again provide 8 channels on a single fiber CWDM uses the

entire frequency band between second and third transmission window (1310/1550 nm

respectively) including both windows (minimum dispersion window and minimum

attenuation window) but also the critical area where OH scattering may occur,

recommending the use of OH-free silica fibers in case the wavelengths between second

and third transmission window shall also be used. Avoiding this region, the channels 31,

49, 51, 53, 55, 57, 59, 61 remain and these are the most commonly used.

13

WDM, DWDM and CWDM are based on the same concept of using multiple

wavelengths of light on a single fiber, but differ in the spacing of the wavelengths, number

of channels, and the ability to amplify the multiplexed signals in the optical space. EDFA

provide an efficient wideband amplification for the C-band, Raman amplification adds a

mechanism for amplification in the L-band. For CWDM wideband optical amplification is

not available, limiting the optical spans to several tens of kilometers.

2.2 COARSE WAVELENGTH DIVISION MULTPLEXING

Originally, the term "coarse wavelength division multiplexing" was fairly generic,

and meant a number of different things. In general, these things shared the fact that the

choice of channel spacings and frequency stability was such that erbium doped fiber

amplifiers (EDFAs) could not be utilized. Prior to the relatively recent ITU standardization

of the term, one common meaning for coarse WDM meant two (or possibly more) signals

multiplexed onto a single fiber, where one signal was in the 1550 nm band, and the other in

the 1310 nm band

.

Fig 2.1 CWDM Bandwidth

In 2002 the ITU standardized a channel spacing grid for use with CWDM (ITU-T

G.694.2), using the wavelengths from 1270 nm through 1610 nm with a channel spacing of

20 nm. (G.694.2 was revised in 2003 to shift the actual channel centers by 1, so that strictly

speaking the center wavelengths are 1271 to 1611 nm.) Many CWDM wavelengths below

14

1470 nm are considered "unusable" on older G.652 specification fibers, due to the

increased attenuation in the 1270–1470 nm bands. Newer fibers which conform to the

G.652.C and G.652.D standards, such as Corning SMF-28e and Samsung Widepass nearly

eliminate the "water peak" attenuation peak and allow for full operation of all 20 ITU

CWDM channels in metropolitan networksTheEthernetLX-4 10 Gbit/s physical layer

standard is an example of a CWDM system in which four wavelengths near 1310 nm, each

carrying a 3.125 gigabit-per-second (Gbit/s) data stream, are used to carry 10 Gbit/s of

aggregate data.

The main characteristic of the recent ITU CWDM standard is that the signals are

not spaced appropriately for amplification by EDFAs. This therefore limits the total

CWDM optical span to somewhere near 60 km for a 2.5 Gbit/s signal, which is suitable for

use in metropolitan applications. The relaxed optical frequency stabilization requirements

allow the associated costs of CWDM to approach those of non-WDM optical components.

CWDM is also being used in cable television networks, where different

wavelengths are used for the downstream and upstream signals. In these systems, the

wavelengths used are often widely separated, for example the downstream signal might be

at 1310 nm while the upstream signal is at 1550 nm.

2.3 DENSE WAVELENGTH DIVISION MULTPLEXING

The EDFAs cost is thus leveraged across as many Dense wavelength division

multiplexing, or DWDM for short, refers originally to optical signals multiplexed within

the 1550 nm band so as to leverage the capabilities (and cost) of erbium doped fiber

amplifiers (EDFAs), which are effective for wavelengths between approximately 1525–

1565 nm (C band), or 1570–1610 nm (L band). EDFAs were originally developed to

replace SONET/SDH optical-electrical-optical (OEO) regenerators, which they have made

practically obsolete. EDFAs can amplify any optical signal in their operating range,

regardless of the modulated bit rate.

15

Fig 2.2 DWDM Bandwidth

In terms of multi-wavelength signals, so long as the EDFA has enough pump

energy available to it, it can amplify as many optical signals as can be multiplexed into its

amplification band modulation format). EDFAs therefore allow a single-channel optical

link to be upgraded in bit rate by replacing only equipment at the ends of the link, while

retaining the existing EDFA or series of EDFAs through a long haul route. Furthermore,

single-wavelength links using EDFAs can similarly be upgraded to WDM links channels

as can be multiplexed into the 1550 nm band.

16

CHAPTER 3

FIBERS USED IN DWDM

3.1 NONZERO DISPERSION SHIFTED FIBER (ITU-T G.655)

Using nonzero dispersion-shifted fiber (NZDSF) can mitigate nonlinear

characteristics. NZDSF fiber overcomes these effects by moving the zero-dispersion

wavelength outside the 1550-nm operating window. The practical effect of this is to have a

small but finite amount of chromatic dispersion at 1550 nm, which minimizes nonlinear

effects, such as FWM, SPM, and XPM, which are seen in the dense wavelength-division

multiplexed (DWDM) systems without the need for costly dispersion compensation. There

are two fiber families called nonzero dispersion (NZD+ and NZD–), in which the zero-

dispersion value falls before and after the 1550-nm wavelength, respectively. The typical

chromatic dispersion for G.655 fiber at 1550 nm is 4.5 ps/nm-km. The attenuation

parameter for G.655 fiber is typically 0.2 dB/km at 1550 nm, and the PMD parameter is

less than 0.1ps/km. The Corning LEAF fiber is an example of an enhanced G.655 fiber

(shown in fig.) with a 32 percent larger effective area

3.2 RECOMMENDATION G.655

The correlation of the measured values of lc, lcc, and lcj depends on the specific

fiber and cable design and the test conditions. While in general, lcc<lcj<lc, a general

quantitative relationship cannot be easily established. The importance of ensuring single-

mode transmission in the minimum cable length between joints at the minimum operating

wavelength is paramount. This may be performed by recommending the maximum cable

cut-off wavelength lcc of a cabled single-mode fiber to be 1480 nm, or for typical jumpers

by recommending a maximum jumper cable cut-off to be 1480 nm, or for worst case

length and bends by recommending a maximum fiber cut-off wavelength to be 1470 nm.

17

Fig 3.1.1550 nm Band Performance

The loss increase for 100 turns of fibre, loosely wound with 37.5 mm radius and

measured at 1550 nm, shall not exceed 0.5 dB For SDH and WDM applications, the fibre

may be used at wavelengths exceeding 1550 nm. The 0.5 dB maximum loss shall apply at

the maximum wavelength of anticipated use (i.e. wavelengths £ 1580 nm). The loss at this

wavelength may be projected from a loss measurement at 1550 nm, using either spectral

loss modeling or a statistical database for that particular fibre design. Alternatively, a

qualification test at the longer wavelength may be performed.

18

CHAPTER 4

DWDM SYSTEMS

At this stage, a basic DWDM system contains several main

components:

1. A DWDM terminal multiplexer. The terminal multiplexer actually contains one

wavelength converting transponder for each wavelength signal it will carry. The

wavelength converting transponders receive the input optical signal (i.e., from a

client-layer SONET/SDH or other signal), convert that signal into the electrical

domain, and retransmit the signal using a 1550 nm band laser. (Early DWDM

systems contained 4 or 8 wavelength converting transponders in the mid-1990s. By

2000 or so, commercial systems capable of carrying 128 signals were available.)

The terminal mux also contains an optical multiplexer, which takes the various

1550 nm band signals and places them onto a single fiber (e.g. SMF-28 fiber). The

terminal multiplexer may or may not also support a local EDFA for power

amplification of the multi-wavelength optical signal.

2. An intermediate line repeater is placed approx. every 80 – 100 km for

compensating the loss in optical power, while the signal travels along the fiber. The

signal is amplified by an EDFA, which usually consists of several amplifier stages.

3. An intermediate optical terminal or optical add-drop multiplexer. This is a

remote amplification site that amplifies the multi-wavelength signal that may have

traversed up to 140 km or more before reaching the remote site. Optical diagnostics

and telemetry are often extracted or inserted at such a site, to allow for localization

of any fiber breaks or signal impairments. In more sophisticated systems (which are

no longer point-to-point), several signals out of the multiwavelength signal may be

removed and dropped locally.

4. A DWDM terminal de-multiplexer. The terminal de-multiplexer breaks the multi-

wavelength signal back into individual signals and outputs them on separate fibers

for client-layer systems (such as SONET/SDH) to detect. However, in order to

allow for transmission to remote client-layer systems (and to allow for digital

domain signal integrity determination) such de-multiplexed signals are usually sent

to O/E/O output transponders prior to being relayed to their client-layer systems.

19

Often, the functionality of output transponder has been integrated into that of input

transponder, so that most commercial systems have transponders that support bi-

directional interfaces on both their 1550-nm (i.e., internal) side, and external (i.e.,

client-facing) side. Transponders in some systems supporting 40 GHz nominal

operation may also perform forward error correction (FEC) via 'digital wrapper'

technology, as described in the ITU-TG.709 standard.

5. Optical Supervisory Channel (OSC). This is an additional wavelength usually

outside the EDFA amplification band (at 1510 nm, 1620 nm, 1310 nm or another

proprietary wavelength). The OSC carries information about the multi-wavelength

optical signal as well as remote conditions at the optical terminal or EDFA site. It is

also normally used for remote software upgrades and user (i.e., network operator)

Network Management information. It is the multi-wavelength analogue to

SONET's DCC (or supervisory channel). ITU standards suggest that the OSC

should utilize an OC-3 signal structure, though some vendors have opted to use 100

megabit Ethernet or another signal format. Unlike the 1550 nm band client signal-

carrying wavelengths, the OSC is always terminated at intermediate amplifier sites,

where it receives local information before retransmission.

Fig 4.1 DWDM Transmission

20

The introduction of the ITU-T G.694.1 frequency grid in 2002 has made it easier to

integrate WDM with older but more standard SONET/SDH systems. WDM wavelengths

are positioned in a grid having exactly 100 GHz (about 0.8 nm) spacing in optical

frequency, with a reference frequency fixed at 193.10 THz (1552.52 nm). The main grid is

placed inside the optical fiber amplifier bandwidth, but can be extended to wider

bandwidths. Today's DWDM systems use 50 GHz or even 25 GHz channel spacing for up

to 160 channel operation.

DWDM systems have to maintain more stable wavelength or frequency than those

needed for CWDM because of the closer spacing of the wavelengths. Precision

temperature control of laser transmitter is required in DWDM systems to prevent "drift"

off a very narrow frequency window of the order of a few GHz. In addition, since DWDM

provides greater maximum capacity it tends to be used at a higher level in the

communications hierarchy than CWDM, for example on the Internet backbone and is

therefore associated with higher modulation rates, thus creating a smaller market for

DWDM devices with very high performance levels. These factors of smaller volume and

higher performance result in DWDM systems typically being more expensive than

CWDM.

Recent innovations in DWDM transport systems include pluggable and software-

tunable transceiver modules capable of operating on 40 or 80 channels. This dramatically

reduces the need for discrete spare pluggable modules, when a handful of pluggable

devices can handle the full range of wavelengths.

4.1 MANAGING THE DWDM NETWORK

4.1.1 Wavelength Converting Transponders

At this stage, some details concerning Wavelength Converting Transponders

should be discussed, as this will clarify the role played by current DWDM technology as

an additional optical transport layer. It will also serve to outline the evolution of such

systems over the last 10 or so years.

As stated above, wavelength converting transponders served originally to translate

the transmit wavelength of a client-layer signal into one of the DWDM system's internal

21

wavelengths in the 1550 nm band (note that even external wavelengths in the 1550 nm will

most likely need to be translated, as they will almost certainly not have the required

frequency stability tolerances nor will it have the optical power necessary for the system's

EDFA).

In the mid-1990s, however, wavelength converting transponders rapidly took on

the additional function of signal regeneration. Signal regeneration in transponders quickly

evolved through 1R to 2R to 3R and into overhead-monitoring multi-bit rate 3R

regenerators. These differences are outlined below:

1R:

Retransmission: Basically, early transponders were "garbage in garbage

out" in that their output was nearly an analogue 'copy' of the received optical signal,

with little signal cleanup occurring. This limited the reach of early DWDM systems

because the signal had to be handed off to a client-layer receiver (likely from a

different vendor) before the signal deteriorated too far. Signal monitoring was

basically confined to optical domain parameters such as received power.

2R:

Re-time and re-transmit: Transponders of this type were not very common

and utilized a quasi-digital Schmitt-triggering method for signal clean-up. Some

rudimentary signal quality monitoring was done by such transmitters that basically

looked at analogue parameters.

3R:

Re-time, re-transmit, re-shape. 3R Transponders were fully digital and

normally able to view SONET/SDH section layer overhead bytes such as A1 and

A2 to determine signal quality health. Many systems will offer 2.5 Gbit/s

transponders, which will normally mean the transponder is able to perform 3R

regeneration on OC-3/12/48 signals, and possibly gigabit Ethernet, and reporting

on signal health by monitoring SONET/SDH section layer overhead bytes. Many

transponders will be able to perform full multi-rate 3R in both directions. Some

vendors offer 10 Gbit/s transponders, which will perform Section layer overhead

monitoring to all rates up to and including OC-192.

22

4.1.2 Muxponder

The muxponder (from multiplexed transponder) has different names depending on

vendor. It essentially performs some relatively simple time division multiplexing of lower

rate signals into a higher rate carrier within the system (a common example is the ability to

accept 4 OC-48s and then output a single OC-192 in the 1550 nm band). More recent

muxponder designs have absorbed more and more TDM functionality, in some cases

obviating the need for traditional SONET/SDH transport equipment.

4.1.3 Reconfigurable Add Drop Multiplexer (ROADM)

As mentioned above, intermediate optical amplification sites in DWDM systems

may allow for the dropping and adding of certain wavelength channels. In most systems

deployed as of August 2006 this is done infrequently, because adding or dropping

wavelengths requires manually inserting or replacing wavelength-selective cards. This is

costly, and in some systems requires that all active traffic be removed from the DWDM

system, because inserting or removing the wavelength-specific cards interrupts the multi-

wavelength optical signal.

With a ROADM, network operators can remotely reconfigure the multiplexer by

sending soft commands. The architecture of the ROADM is such that dropping or adding

wavelengths does not interrupt the 'pass-through' channels. Numerous technological

approaches are utilized for various commercial ROADMs, the tradeoff being between cost,

optical power, and flexibility.

4.1.4 Optical Cross Connects (OXCs)

When the network topology is a mesh, where nodes are interconnected by fibers to

form an arbitrary graph, an additional fiber interconnection device is needed to route the

signals from an input port to the desired output port. These devices are called optical cross-

connectors (OXCs).Various categories of OXCs include elctronic, optical,and wavelength

selective devices.

23

CHAPTER 5

PRACTICAL DWDM SYSTEM

5.1 DWDM SETUP

Fig 5.1 DWDM Setup

5.1.1 Components in DWDM

DWDM setup mainly consists of the following blocks

1.Add/Drop Multiplexer (ADM)

2.Transponders (TRP)

3.Multiplexer

4.Optical Booster Amplifier (OBA)

5.Optical Supervisory Channel (OSC)

6.Optical Power Amplifier (OPA)

7.De-multiplexer

24

5.1.2 Procedure for Setup

The optical input signal is given to the ADM whose input and output are black and

white signals. From the TX of ADM, the signals are given to the TX IN of TRP. The

wavelength will depend on whether the DWDM is a 2.5 or 10 Gbps system. The TX OUT

of TRP will be coloured signals with different colour for different wavelength. From the

TX OUT, connection is given to IN of MUX. It has different ports. It combines different

signal inputs to form a single signal. The OUT of MUX will be common to all ports. This

OUT is given to IN of OBA.

The OBA contains EDFA which provides high power amplification. Additional

OSC signals are given to the OBA which is useful for supervising the signal and

eliminating the unnecessary signals. From OUT of OBA, the signal is passed to the line

fiber. From the other end of the line fiber, signals are given for amplification to OPA IN.

Here the signal gets amplified and the OSC signals are dropped. Thus, from OPA OUT,

only the information carrying signal will be given to DEMUX IN. In DEMUX single

signals is converted to different signals. This is then fed to the RX IN of TRP. The out

coming black and white signals from RX OUT of TRP the signals are given to ADM RX.

Thus the complete transmission and reception of signals are shown within the DWDM.

25

CHAPTER 6

FORWARD ERROR CORRECTION

Forward error correction (FEC) or channel coding is a technique used

for controlling errors in data transmission over unreliable or noisy communication

channels. The central idea is the sender encodes their message in a redundant way by using

an error-correcting code (ECC). The American mathematician Richard Hamming

pioneered this field in the 1940s and invented the first error-correcting code in 1950:

the Hamming (7,4) code.

FEC processing in a receiver may be applied to a digital bit stream or in the

demodulation of a digitally modulated carrier. For the latter, FEC is an integral part of the

initial analog-to-digital conversion in the receiver. The Viterbi decoder implements a soft-

decision algorithm to demodulate digital data from an analog signal corrupted by noise.

Many FEC coders can also generate a bit-error rate (BER) signal which can be used as

feedback to fine-tune the analog receiving electronics.

FEC is accomplished by adding redundancy to the transmitted information using a

predetermined algorithm. A redundant bit may be a complex function of many original

information bits. The original information may or may not appear literally in the encoded

output; codes that include the unmodified input in the output are systematic, while those

that do not are non-systematic. Many FEC coders can also generate a bit-error rate (BER)

signal which can be used as feedback to fine-tune the analog receiving electronics.

6.1 TYPES OF FEC

Forward error correction (FEC) or channel coding is a technique used

for controlling errors in data transmission over unreliable or noisy communication

channels. The central idea is the sender encodes their message in a redundant way by using

an error-correcting code (ECC). The American mathematician Richard Hamming

pioneered this field in the 1940s and invented the first error-correcting code in 1950:

the Hamming (7,4) code.

FEC processing in a receiver may be applied to a digital bit stream or in the

demodulation of a digitally modulated carrier. For the latter, FEC is an integral part of the

initial analog-to-digital conversion in the receiver. The Viterbi decoder implements a soft-

decision algorithm to demodulate digital data from an analog signal corrupted by noise.

26

Many FEC coders can also generate a bit-error rate (BER) signal which can be used as

feedback to fine-tune the analog receiving electronics.

FEC is accomplished by adding redundancy to the transmitted information using a

predetermined algorithm. A redundant bit may be a complex function of many original

information bits. The original information may or may not appear literally in the encoded

output; codes that include the unmodified input in the output are systematic, while those

that do not are non-systematic. Many FEC coders can also generate a bit-error rate (BER)

signal which can be used as feedback to fine-tune the analog receiving electronics.

6.1.1 List Of Few Error Correcting Codes

Convolutional Code

Group Codes

Hamming Code

Lexicographic Code

Long Code

M of n Codes

Reed-Solomon Error Correction

Reed-Muller Code

Turbo Code

Trellis Coded Modulation

6.2 FEC IN OPTICAL NETWORK

Light traveling over fiber optic connections must contend with natural impairments

that degrade signal quality. The amount of degradation increases with both distance and

data rate. One way to reduce the effects of the impairments is to utilize forward error

correction (FEC). Now that FEC can be implemented readily in inexpensive silicon, it has

become an essential component of optical transport networking (OTN) equipment,

particularly at today’s higher frequencies. Achieving higher gain by using FEC algorithms

reduces carrier Capex since it allows optical devices to be spaced further apart in networks.

Of course, the cost of implementing FEC does counter the gains.

There are many ways to implement FEC. The ITU-T (Recommendation G.709)

defined a standard method for OTN frames utilizing a Reed-Solomon technique. The

approach, sometimes called Generic FEC (GFEC), is necessary for any standards-

compliant OTN framer. However, the method is satisfactory only for lower-data rate and

27

shorter reach applications. Researchers have developed more advanced FEC techniques,

optimized to achieve higher gain. Some of the algorithms are proprietary, while others

have been standardized. Various Enhanced FEC (EFEC) techniques built for higher gain

have been defined within ITU-T (Recommendation G.975.1). It outlines nine techniques

(I.1 through I.9) which can be used in transponders, regenerators, muxponders, and

switches at OTU-2 (10Gbps) and OTU-3 (40Gbps). However, equipment deployed at

OTU-4 (100Gbps) must utilize proprietary FEC as well as soft-decision FEC (SDFEC)

techniques.

Optical equipment manufacturers that have deployed OTN technologies have used

a variety of FEC technologies in OTU-2, OTU-3, and OTU-4 cards. The choice has often

varied even within a single company with one division selecting an ASSP provider with a

specific FEC algorithm for one OTN card, while another division has developed a card

utilizing a different algorithm. As OEMs integrate multiple platforms, they prefer to

develop cards supporting Universal FEC, allowing a direct interface to a variety of

techniques, depending on which card is attached at the other end of the optical fiber. One

way of supporting universal FEC is to integrate multiple techniques in silicon and select

the appropriate one upon deployment. The disadvantage of this approach is that it wastes

silicon since only one part of the device is actually used in practice. Another option is to

use programmable logic and configure the silicon at deployment with the appropriate FEC

technology.

Field programmable gate array (FPGA) suppliers are investing more aggressively

in OTN IP and offering the capabilities of delivering FEC, framer, and muxing solutions

optimized for a specific application. Vendors developing their own FEC techniques for

next-generation systems can offer configurable support for standards-based techniques

without using additional silicon. As bandwidth grows and OTN deployments evolve to

100Gbps and beyond, the utilization of FEC techniques will become more widespread.

Implementing the front-end FEC with programmable logic allows for a risk-proof

approach that supports a variety of algorithms and allows for future needs, new standards,

and further developments.

28

CHAPTER 7

SPECTRUM SCLICED DWDM TRANSMISSION

7.1 CONFIGURATION OF TARGET SYSTEMS

Fig 7.1 illustrates the theoretical models used to clarify the scalability of

1.25/10-G spectrum-sliced DWDM transmission systems. In model 1, the signal is

received by an avalanche photodiode (APD) receiver, while model 2 uses an optical

preamplifier receiver.

All transmitters (Tx.) have the same broadband incoherent light sources, such as

the light-emitting diode (LED), SOA, SLD, and erbium-doped fiber amplifier (EDFA).

The output signals from each transmitter are spectrally sliced at different wavelengths and

multiplexed through the MUX. The multiplexed signals are then transmitted to be de-

multiplexed in the de-multiplexer (DEMUX). The DEMUX uses the same filter as the

MUX. For example, an arrayed waveguide grating (AWG) or a thin film WDM filter can

be used as the MUX/DEMUX.

Fig 7.1 Model For Theoretical Analysis

29

In the case of model 2, the multiplexed signals are amplified

simultaneously before passing through the DEMUX. The loss budget is defined as the

allowable loss in the transmission line in both cases.

7.2 THEORETICAL ANALYSIS

Designing a WDM transmission system involves both the loss budget and the

number of channels. However, in WDM transmission systems that use a spectrum-slicing

scheme, there is a tradeoff between these two requirements. To increase the number of

channels, it is necessary to narrow the channel spacing due to the fixed bandwidth of the

light sources. However, as the amount of signal-signal beat noise, which is the dominant

noise factor, is inversely proportional to the sliced bandwidth, narrowing the channel

spacing degrades the received sensitivity, and the loss budget becomes smaller.

In this section, the scalability of 1.25/10-G spectrum-slice DWDM transmission is

discussed taking the above two requirements into account. We first clarify the scalability

of the models shown in Fig. 1 and then study the possibility of using FEC to enhance this

scalability. The results of experiments conducted to prove the validity of the theoretical

analysis are also presented.

7.2.1 Scalability: Tradeoff Between Loss Budget And Channel Spacing

Line (1) and (2) in Fig 7.2 show the received sensitivity for the bit error rate (BER)

of 1E-12 against the sliced bandwidth in 1.25-G spectrum-sliced systems. The sliced

bandwidth represents the width of MUX/DEMUX with rectangular profile. The broadband

light sources were assumed to be unpolarized. Line (1) corresponds to the case of an APD

receiver, and line (2) is the one of an optical preamplifier receiver. Line (3) shows the

effective output optical power against the sliced bandwidth when the power densities of

broadband light sources were assumed to be −10 dBm/nm. The effective optical power was

determined as the power per one channel after passing through the MUX. The loss budget

can be derived from the difference between lines (1)–(3). As filter losses are not

considered in this discussion for simplicity, the loss budget can be said as the allowable

loss in the transmission line.

30

Fig 7.2 Calculated loss budget to achieve the BER of 1E-12 without FEC (1.25 Gb/s per

channel).

Fig 7.3 shows the calculated loss budget of 10-G spectrum sliced systems. Lines

(1)–(3) represent the same ones as in Fig. 2, respectively. It is found that the required

minimum sliced bandwidth is around 350 GHz. When the sliced bandwidth is set to 360

GHz, which is the typical value for systems of 600-GHz (4.8 nm) channel spacing, the

maximum number of channel are no more than eight, assuming broadband light sources

with the bandwidth of 40 nm. Moreover, the loss budget is only 7 dB in the case of the

APD receiver, and it cannot be expanded to more than 13 dB, even with the optical

preamplifier receiver.Thus, it is obvious that some additional attempts are needed to realize

high data rate DWDM transmission systems using the spectrum-sliced scheme.

31

Fig 7.3 Calculated loss budget to achieve the BER of 1 × 10−12 without FEC (10 Gb/s

per channel).

7.2.2 Use of FEC for Expanding The Scalability

As discussed above, the scalability of spectrum-sliced DWDM transmission

systems for high data rate is limited severely. Therefore, it is difficult to design systems

with large loss budget and a large enough number of channels.

To expand the scalability, we study the use of FEC, which has been usually used to

improve the received sensitivity in the long-haul transmission systems. We assumed a code

that can improve the BER of 1.8E−4 to the BER of 1E−12,which is a feature that a well-

known Reed–Solomon (255,239) code can achieve. In our study, as an example to reveal

the general effect FEC offers, a Reed–Solomon (255,239) code which redundancy ratio is

1/14 was chosen.

32

Fig 7.4 Calculated loss budget to achieve the BER of 1E−12 with FEC (10 Gb/s per

channel).

Fig. 7.4 shows the calculated loss budget to achieve the BER of 1 × 10−12 with

FEC in 10-G spectrum-slice DWDM transmission systems. Line (1) is the case of an APD

receiver, and line (2) is the one of an optical preamplifier receiver. In both cases, the

number of polarization modes of broadband light sources was assumed to be 2. Line (3)

shows the effective output optical power against the sliced bandwidth when the power

densities of broadband light sources were assumed to be −10 dBm/nm.

Comparing with Fig 7.3, one can see the received sensitivity is improved owing to

the effect of FEC. For example, at the sliced bandwidth of 1000 GHz, the improvement is

4/6 dB when received with the APD receiver and the optical preamplifier receiver,

respectively. In addition to this effect, FEC offers one more effect. As the required BER to

the received data changes from 1E−12 to 1.8E−4, the allowable maximum amount of noise

33

increases, and when the signal–signal beat noise is the dominant noise factor, the increase

of the amount of noise permits a narrower sliced bandwidth. The required minimum sliced

bandwidth narrows from 350 to 95 GHz.

From Fig 7.4, we can see that the loss budget of 22 dB with the sliced bandwidth of

120 GHz can be obtained when the optical preamplifier receiver is used. If we assume that

the bandwidth of broadband light sources is 40 nm, we can design a 200-GHz channel

spacing, 25-channel, 10-G spectrum-sliced DWDM transmission system with the loss

budget of 22 dB by using FEC and the optical preamplifier.

Fig 7.5 Expansion of scalability permitted by using FEC.

Fig 7.5 concludes the scalability of spectrum-sliced DWDM transmission systems.

Loss budgets against the data rate of 1.25, 2.5, and 10 Gb/s are shown. Lines (1) and (2)

correspond to the case when an APD receiver is used and the time channel spacing is

34

200/600 GHz, respectively. Lines (3) and (4) are the cases when the FEC-coded signal

whose channel spacing is 200 GHz is received with an APD receiver and an optical

preamplifier receiver, respectively. The sliced bandwidth was assumed to be 0.6 times of

channel spacing (the same as that used in the previous analysis). The values of 1.25 and 10

Gb/s are extracted from Figs. 2–4, and the ones of 2.5 Gb/s are newly calculated.

As can be seen from Fig 7.5, when the APD receiver is used, the 10-Gb/s signal

cannot be supported without broadening the channel spacing to 600 GHz (in other words,

the sliced bandwidth to 360 GHz). It is found that by using FEC, the 10-Gb/s signal with

200-GHz channel spacing comes to be able to be supported by the APD receiver. The

results indicate that the usage of FEC not only increases the loss budget but also increases

the number of available channels. It can also be seen that the usage of the optical amplifier

receiver in addition to FEC further increase the scalability so that the loss budget over 20

dB can be achievable for 10-Gb/s signals with 200-GHz channel spacing.

Fig 7.6 Experimental setup for confirming the results of theoretical analysis.

35

Fig 7.7

BER

characteristics

when sliced bandwidth is 130/200 GHz.

We verified the validity of our theoretical analysis with the experiment, whose

setup is shown in Fig 7.6. An unpolarized broadband incoherent signal was spectrally

sliced with a filter, and the BER versus the received power was measured using an optical

preamplifier receiver. The unpolarized signal was produced by externally modulating the

ASE of a polarization-insensitive SOA with a pseudorandom signal (2^23 − 1) at the rate

of 10.7 Gb/s that concerns the redundancy of Reed–Solomon (255,239) code. A

polarization-insensitive electro-absorption (EA) modulator was used as the external

modulator. The slice filter corresponds to the MUX in the model 2 in Fig 7.1. The optical

preamplifier receiver consisted of an optical amplifier, a filter, a photo-diode (PD), and a

clock-data recovery (CDR) circuit. As the optical amplifier, an EDFA was used. The noise

figure (NF) of the EDFA was 7 dB, which is the same value used in the theoretical

analysis. The filter corresponds to the DEMUX in the model 2 in Fig 7.1. Two filters used

in this experiment had the same bandwidth, and their spectral profiles were nearly

rectangular. We measured the BER characteristics when the sliced bandwidths were 130

and 200 GHz.

36

Fig 7.7 shows the BER characteristics. The open squares/ circles plot the measured

values when the sliced bandwidth is 130/200 GHz, respectively. The closed squares/circles

represent the values gained when the received data are assumed to be decoded. The

required received power for the BER of 1E−12 was −32/ − 34 dBm when the sliced

bandwidth was 130/200 GHz. These measured values agree well with the result of the

theoretical analysis shown in Fig 7.4. As the differences between measured and calculated

results were less than 1 dB, we could verify the validity of our theoretical analysis in the

range of the sliced bandwidth between 130 and 200 GHz.

37

CHAPTER 8

DWDM EXPERIMENTS

In this section, we describe the results of two spectrum sliced DWDM transmission

experiments. One confirms 10-Gb/s, eight channel DWDM transmission without FEC and

the channel spacing of 200 GHz. The other assesses 10-Gb/s, eight channel DWDM

transmission with FEC and the channel spacing of 200 GHz. We have also conducted 100

GHZ Grid 40 Channel, 50 GHZ Grid 80 Channel, 25 GHZ Grid 160 Channel DWDM

Transmission without FEC. We have also conducted 100 GHZ Grid 40 Channel, 50 GHZ

Grid 80 Channel, 25 GHZ Grid 160 Channel DWDM Transmission with FEC.

8.1 10-G SPECTRUM SCLICED DWDM TRANSMISSION USING FEC

Fig 8.1 (1) Experimental setup for 10 G × eight ch DWDM transmission; (2) measured

optical spectrum after multiplexing.

38

An eight channel 10-G spectrum-sliced WDM transmission was conducted with the

setup shown in Fig 8.1(1). The signal for the measurement channel was launched from the

same polarization-insensitive broadband light source used in the experiment shown in Fig

7.6. This signal was pseudorandom (2^23 − 1), and its data rate was 10.7 Gb/s to consider

the effectiveness of FEC. An eye diagram is shown in the inset of Fig 8.1(1); a clear eye

opening can be observed. Dummy signals were produced by externally modulating the

ASE of an EDFA with a polarization-insensitive EA modulator and split by a 1 : 8 coupler

after being amplified by an EDFA. The measurement signal and seven out of eight dummy

signals were spectrally sliced, multiplexed through an 8-port thin-film WDM filter with a

nearly rectangular profile and transmitted. Fig 8.1(2) shows a spectrum of the multiplexed

signals. All signals have nearly the same peak power of −19 dBm and a 3-dB bandwidth of

130 GHz. The transmitted signals were received with an optical preamplifier receiver

constructed around an EDFA whose NF was 5.5 dB, an 8-port thin-film WDM filter, a PD,

and a CDR circuit. The thin-film WDM filter was the same one used for multiplexing, and

it not only de-multiplexed the multiplexed signals but also eliminated the ASE of the

preamplifier. We measured the BER characteristics when the transmission line was 20 km

of dispersion shift fiber (DSF) or 2 km of SMF.

39

CHAPTER 9

RESULTS, ANALYSIS AND DISCUSSIONS

Fig 9.1 BER characteristics of 10 Gb/s Eight Channel DWDM Transmission without FEC.

Fig 9.1 Shows BER Characteristics of 10 Gb/s Eight Channel DWDM

Transmission without FEC. BER is measured to be 8E-4 when the received power is -34.5

dB m.

0.00E+00

1.00E-04

2.00E-04

3.00E-04

4.00E-04

5.00E-04

6.00E-04

7.00E-04

8.00E-04

9.00E-04

-40 -30 -20 -10 0

BER

Recieved Power (dBm)

10 GB/s Eight Channel DWDM Transmission without FEC

BER SMF 2KM without FEC

BER DSF20KM without FEC

40

Fig 9.2 BER characteristics of 10 Gb/s Eight Channel DWDM Transmission with FEC.

Fig 9.2 Shows BER Characteristics of 10 Gb/s Eight Channel DWDM

Transmission with FEC. BER is measured to be 1E-12 when the received power is -34.5

dB m.

0.00E+00

1.00E-12

2.00E-12

3.00E-12

4.00E-12

5.00E-12

6.00E-12

7.00E-12

-38 -37 -36 -35 -34 -33 -32

BER

Recieved Power(dBm)

10 Gb/s Eight Channel DWDM Transmission with FEC

BER SMF2Km(FEC)

BER DSF 20Km(FEC)

41

Fig. 9.3. BER characteristics of 40 Channel 100GHZ spacing DWDM Transmission

without FEC.

Fig 9.3 Shows BER Characteristics of 40 Channel 100 GHZ Spacing DWDM

Transmission without FEC. BER is measured to be 8E-5 when the received power is -35

dB m.

Fig. 9.4. BER characteristics of 40 Channel 100GHZ spacing DWDM Transmission with

FEC

0.00E+00

1.00E-05

2.00E-05

3.00E-05

4.00E-05

5.00E-05

6.00E-05

7.00E-05

8.00E-05

9.00E-05

-40 -30 -20 -10 0

BER

Recieved Optical Power(dBm)

100 GHZ grid 40 Channel DWDM Transmission without FEC

BER without FEC

0.00E+00

1.00E-13

2.00E-13

3.00E-13

4.00E-13

5.00E-13

6.00E-13

-38 -37 -36 -35 -34 -33 -32

BER

Recieved Power(dBm)

100 GHZ Grid 40 Channel DWDM Transmission With FEC

BER (FEC)

42

Fig 9.4 Shows BER Characteristics of 40 Channel 100 GHZ Spacing DWDM

Transmission with FEC. BER is measured to be 1E-13 when the received power is -35 dB

m

Fig 9.5 BER characteristics of 80 Channel 50GHZ spacing DWDM Transmission without

FEC

Fig 9.5 Shows BER Characteristics of 80 Channel 50 GHZ Spacing DWDM

Transmission without FEC. BER is measured to be 8E-7 when the received power is -35

dB m.

0.00E+00

1.00E-07

2.00E-07

3.00E-07

4.00E-07

5.00E-07

6.00E-07

7.00E-07

8.00E-07

9.00E-07

-40 -30 -20 -10 0

BER

Recieved Power (dBm)

50 GHz Grid DWDM Transmission without FEC

BER without FEC

43

Fig 9.6 BER characteristics of 80 Channel 50GHZ spacing DWDM Transmission with

FEC

Fig 9.6 Shows BER Characteristics of 80 Channel 50 GHZ Spacing DWDM

Transmission without FEC. BER is measured to be 4E-14 when the received power is -35

dB m.

Fig 9.7 BER characteristics of 160 Channel 25GHZ spacing DWDM Transmission

without FEC

0.00E+00

1.00E-14

2.00E-14

3.00E-14

4.00E-14

5.00E-14

6.00E-14

7.00E-14

8.00E-14

9.00E-14

-38 -37 -36 -35 -34 -33 -32

BER

Recieved Power (dBm)

50 GHZ Grid 80 Channel DWDM Transmission with FEC

BER (FEC)

0.00E+00

1.00E-08

2.00E-08

3.00E-08

4.00E-08

5.00E-08

6.00E-08

7.00E-08

8.00E-08

9.00E-08

-40 -30 -20 -10 0

BER

Recieved Power(dBm)

25 GHZ Grid 160 Channel DWDM Transmission without FEC

BER without FEC

44

Fig 9.7 Shows BER Characteristics of 160 Channel 25 GHZ Spacing DWDM

Transmission without FEC. BER is measured to be 8E-8 when the received power is -35

dB m.

Fig 9.8 BER characteristics of 160 Channel 25GHZ spacing DWDM Transmission with

FEC

Fig 9.8 Shows BER Characteristics of 160 Channel 25 GHZ Spacing DWDM

Transmission without FEC. BER is measured to be 1E-14 when the received power is -35

dB m.

0.00E+00

1.00E-14

2.00E-14

3.00E-14

4.00E-14

5.00E-14

6.00E-14

7.00E-14

8.00E-14

9.00E-14

-38 -37 -36 -35 -34 -33 -32

25 GHZ Grid 160 Channel DWDM Transmission with FEC

BER (FEC)

45

TABLE 9.1

BER values of Various DWDM Transmission System

DWDM System BER without FEC BER with FEC

10 Gb/s Eight Channel 8E-4 1E-12

100 GHZ Spacing 40

Channel

8E-5 1E-13

50 GHZ Spacing 80

Channel

8E-7 4E-14

25 GHZ Spacing 160

Channel

8E-8 1E-14

Table 9.1 Shows the BER values of Various DWDM Transmission System Without

FEC and BER Characteristics of Various DWDM Transmission System With FEC.

46

CHAPTER 10

CONCLUSION AND FUTURE SCOPE

10.1 CONCLUSION

This paper clarified the scalability of 1.25/10-G spectrum sliced DWDM

transmission systems through a theoretical analysis; the analysis results were confirmed by

transmission experiments. First, the tradeoff between the loss budget and the sliced

bandwidth (i.e., the number of channels assuming fixed bandwidth light sources) was

elucidated by the theoretical analysis. Next, the expansion of the scalability made possible

by using FEC was studied, and it was revealed that four times as many as channels can be

accommodated with the use of a familiar FEC code. Finally, two WDM transmission

experiments were performed and shown to verify the results of the theoretical analysis.

10.2 FUTURE SCOPE

Super FEC Coding can be implemented in high data rate DWDM Systems with

more number of channels.

47

REFERENCES

[1] Takashi Mitsui, Kazutaka Hara, Masamichi Fujiwara, Jun-ichi Kani Masashi

Tadokoro,Naoto Yoshimoto, and Hisaya Hadama.;‖Simple and Scalable

WDM/TDMA-PON using Spectral Slicing and Forward Error Correction‖ , J.

LightwaveTechnol., 2011.

[2] S. Kaneko et al. ‗Scalability of Spectrum-sliced DWDM transmission and Its

Expansion Using Forward Error Correction„, J. Light wave Technol., 2006, vol. 24,

pp. 1295-1301

[3] “Spectral grids for WDM applications: DWDM frequency grid,” ITU-T

Recommendation G.694.1, Jun. 2002.

[4] K. Fukuchi, T. Kasamatsu, M. Morie, R. Ohhira, T. Ito, K. Sekiya, D. Ogasahara,

and T. Ono, “10.96-Tb/s (273×40-Gb/s) triple-band/ultra dense WDM optical-

repeated transmission experiment,” presented at the Tech. Dig. Optical Fiber

communication Conf. (OFC), Anaheim, CA,2001, Paper PD24.

[5] R. D. Feldman, E. E. Harstead, S. Jiang, T. H. Wood, and M. Zirngibl, “An

evaluation of architectures incorporating wavelength division multiplexing for

broad-band fiber access,” J. Light w. Technol., vol. 16, no. 9,pp. 1546–1559, Sep.

1998.

[6] K. Iwatsuki, J. Kani, H. Suzuki, and M. Fujiwara, “Access and metro networks

based on WDM technologies,” J. Light w. Technol., vol. 22, no. 11, pp. 2623–2630,

Nov. 2004.

[7] ] J. S. Lee, Y. C. Chung, and D. J. DiGiovanni, “Spectrum-sliced fiber amplifier

light source for multichannel WDM applications,” IEEE Photon. Technol. Lett.,

vol. 5, no. 12, pp. 1458–1461, Dec. 1993.

[8] ] K. Akimoto, J. Kani, M. Teshima, and K. Iwatsuki, “Super-dense WDM

transmission of spectrum-sliced incoherent light for wide-area access network,” J.

Light w. Technol., vol. 21, no. 11, pp. 2715–2722, Nov. 2003.

48

[9] J. H. Han, S. J. Kim, and J. S. Lee, “Transmission of 4 × 2.5-Gb/s spectrum-sliced

incoherent light channels over 240 km of dispersion shifted fiber with 200-GHz

channel spacing,” IEEE Photon. Technol. Lett., vol. 11, no. 7, pp. 901–903, Jul.

1999.

[10] S. J. Kim, J. H. Han, J. S. Lee, and C. S. Park, “Intensity noise suppression in

spectrum-sliced incoherent light communication systems using a gain saturated

semiconductor optical amplifier,” IEEE Photon. Technol. Lett., vol. 11, no. 8, pp.

1042–1044, Aug. 1999.

[11] K. H. Han, E. S. Son, H. Y. Choi, K. W. Lim, and Y. C. Chung, “Bidirectional

WDMPON using light-emitting diodes spectrum-sliced with cyclic arrayed-

waveguide grating,” IEEE Photon. Technol. Lett., vol. 16, no. 10,pp. 2380–2382,

Oct. 2004.

49

PUBLICATIONS

[1] S.Immanuel, Dr.A.Sivanantha Raja, (2012) “Increase in Scalability of DWDM

Transmission using Forward Error Correction". National Conference on

Microwave & Optical Communication, Alagappa Chettiar College of Engineering

& Technology, Karaikudi.

50